The wiring is only going to the bell, keypad, and wireless expander. If you have a multimeter and know how to use it then testing if the wiring is good is simple. But the fact that you have to ask how would say to me that you dont.
I wouldnt advise replacing the panel over reviving the old Gardtec stuff, its obsolete for a reason.
As the panel and detection is all wireless, the best solution is to replace it with similar. You could DIY it if you are feeling brave enough (what could possibly go wrong?) . Or you could get a company in to do it properly for you.